Thermotropic liquid crystalline polymers are classified as “rigid rod” polymers as their molecular structure is typically composed of aromatic units linked by ester groups, as well as other groups (e.g., amide groups). The rigid, rod-like structure allows the polymers to exhibit liquid crystalline behavior in their molten state (thermotropic nematic state). Due to the presence of this nematic state in the melt, these materials also exhibit unique rheological properties. One such property is a “shear thinning behavior” characterized by a decrease in complex viscosity with increasing shear rates. This high shear thinning behavior is particularly attractive in the fabrication of parts with intricate geometries (e.g., electrical connectors) because the polymers can flow well under heat and shear to uniformly fill complex parts at fast rates without excessive flashing or other detrimental processing issues. Despite these benefits, the aforementioned polymers still have various drawbacks. For example, the heat resistance of the polymer is often relatively poor as compared to other engineering thermoset materials, as evidenced by a relatively lower deflection temperature under load (“DTUL”). This can lead to inadequate mechanical properties at elevated temperatures, which is particularly problematic as the demand for heat resistance at high temperatures continually increases in molding, fiber, and film applications.
To improve heat resistance and other properties, various attempts have been made to thermally crosslink the polymer while still maintaining its liquid crystal order. In one such process, for example, a thermoset LCP polymer is produced using 4-phenylethynyl phthalic anhydride (“4-PEPA”). While this approach has some advantages, 4-PEPA is a relatively expensive crosslinking agent. Also, the incorporation of the 4-PEPA in LCP as endcaps requires a multi-step process. In the 1st step, either monofunctional phenolic or monofunctional carboxylic acid derivatives of 4-PEPA must initially be synthesized. These monofunctional derivatives are then reacted with LCP monomers to form a low molecular weight oligomer that is functionalized by 4-PEPA end groups. In the final step, the oligomers are heated at temperatures typically ranging from 350° C. to 400° C. to induce thermal crosslinking. While the resulting thermoset resin can exhibit some improved properties, problems nevertheless remain. For instance, the oligomer that is achieved in the 2nd step of the reaction generally has a low molecular weight and melting temperature, which can adversely impact the thermal and mechanical properties of the resulting polymer and restrict the manner in which it may be processed prior to crosslinking. Further, the use of a multi-step reaction to form the thermoset polymer can also be costly and overly complex.
As such, a need exists for an improved technique for forming thermoset liquid crystalline polymers.